Command-line interface support

[irydacea]

Wespal could use a batch/CLI mode for image manipulation.

There are some sub-issues to consider for this:

• Currently, custom colour ranges and palettes need to be defined in the UI as we use the QSettings interface instead of a visible configuration file. We could probably add a way to specify 'transient' (not saved) definitions in the CLI.
• Overriding output filenames/specifying output filenames on a per-task basis may be desirable. e.g.
  --input elynia.png --output RC(magenta>green)=elynia-green.png --output RC(magenta>blue)=elynia-blue.png
• It may be desirable to allow multiple transformations per image file, e.g. swap or recolour more than one palette in the image.
• WML read support would be nice, but it's definitely not worth the hassle — Wesnoth's own config class and WML parser/preprocessor suffer from super-tight coupling with the rest of the game and cannot be imported easily.
• WML write support for colour artefacts may be desirable (the Generate WML option but in CLI form).
• Responses from Wespal should be completely non-interactive so we can do with just a QCoreApplication in this mode instead of initialising the whole kitchen sink.